The paper presents mainly the physical-mechanical properties of the lime-treated Ariake clay with fly ash (LAF), and the utilization as road materials. The effects of quicklime and fly ash on the improvement of the strength of Ariake clay were investigated. Considerable changes in strength of the mixture due to the addition of fly ash were observed which were explained based on the strength mechanism of the LAF mixture. The engineering properties of the LAF mixture were improved due to a certain amount of replacement of lime with fly ash. Results from pavement structure analysis show the mixture can be used as either subgrade or subbase.